The manufacture of electronic integrated circuits relies heavily on the use of image projection techniques to expose resist-coated wafers with light or X-rays. The patterns formed by this exposure determine the various circuit connections and configurations. In any exposure method, accuracy of the projected image is a prime consideration. This accuracy is particularly important in the manufacture of high density random access memories (RAM) in which the yield and ultimately the cost of the components depend heavily on meeting tight exposure placement requirements. With the increasing demand for high performance integrated circuits, the techniques to fabricate semiconductor substrates for microelectronic devices and other purposes have been undergoing continuous development and now include the use of scanning-electron beam lithography systems, both for producing high quality lithographic masks and for direct pattern generation.
Electron beam lithography systems use electron sources that emit electrons at all angles. The electrons are then constrained by the remainder of the system into a narrowly diverging beam. Succeeding lenses then focus the beam into one or more cross-overs before the beam reaches the target. In these systems, electron beams are formed by an electron beam column that, at a minimum, includes an electron source at an object plane and a target at the image plane. Usually the electron beam column includes at least an electron source at the object plane, one or more lenses, one or more apertures, and the target at the image plane. Columns for electron beam lithographic mask exposure include at least an electron source at the object plane, one or more lenses, one or more apertures, one or more deflectors, a set of beam blankers (which can be driven to stop the beam reaching the target), and a target at the image or mask plane.
In direct pattern generation where the electron beam system creates a pattern directly on a chip covered with resist material, the often complicated and time consuming mask-making process is eliminated. However, one of the key economic considerations in a direct electron beam lithography system for a production environment is the throughput achieved by direct writing relative to a system using a series of masks. This is of particular importance, because direct writing is necessarily a series output process. Hence, time constraints become even more critical in direct pattern generation.
As manufacturers seek ever higher writing speeds, other significant problems also appear. These problems arise often as a result of the relationship among these various parameters. For example, as the writing speed increases, the current density must be increased to maintain the same exposure on the resist. However, higher current densities lead to beam broadening due to electron—electron interactions, thereby deleteriously increasing the line width. Also, a shortened exposure time further requires a shortened blanking time, since the rise time of the blanker is closely related to the accuracy of the exposure of each pixel, and is also a major concern in avoiding extraneous exposure during blanking. Hence, blanking time in raster scan type electron beam devices remains one of the key factors limiting throughput.
Electron beam lithography systems use magnetic lenses to provide mainly radial acceleration and deceleration in focusing the beam, which is also at ground potential, at the target. The remainder of the column is held at ground potential unless more electrostatic lenses are used to make the electrons “drift,” i.e., experience no electrical acceleration or deceleration through the remainder of the column.
The electron beam position is conventionally controlled via a technique called raster scanning. In this method, the electron beam is repeatedly deflected in a continuous series of ramp deflections and flyback periods similar to a scanning technique used in televisions. Typically, the electron beam is deflected as rapidly as possible to minimize the time required to completely expose a pattern. This increases the production rate (i.e. throughput) and lowers the unit cost per mask or wafer.
Conventionally, the deflection signal is measured by its product, i.e., the patterns on the mask or wafer. Non-ideal pattern placement seen on the mask or wafer is identified through a series of measurements and tests, and appropriate modifications are then made to the deflection signal to correct these deficiencies. This method of modifying the performance of the deflection signal relies heavily on standard procedures of writing a pattern, processing the mask or wafer, and then measuring the accuracy of the patterns with proven, but relatively slow, metrology techniques. This process may require from one hour for a simple calibration to several weeks for a full calibration.
Preferably, calibration is performed without invoking the time-consuming process of writing and reading actual exposures of masks and wafers. “Real-time” characterization of electron beam parameters is achieved by scanning the electron beam at very low frequencies over a reference grid with known positions. This technique is well-known in the art and provides sufficient information to functionally calibrate an electron beam lithography system.
However, the correlation between these low frequency measurements and the actual beam writing deflection signal is not exact. Specifically, the high frequency writing signal introduces several anomalies in the electron beam which require further characterization via extensive pattern writing and reading. These anomalies include deflection axis crosstalk, slight deflection axis rotation effects, gain differences, and scan offsets, all of which vary as a function of scan frequency.
One important aspect of calibrating scanning electron beam equipment such as scanning electron microscopes and raster scan electron beam lithography machines includes the calibration of the scan amplitude. This is commonly done by reference to the mechanical movement of the stage on which the sample or work piece is mounted. By identifying a feature on the sample and its position within the scan for two positions of the stage where the difference in stage position is known from some measuring device such as a laser interferometer the dimensions of the scan can be determined.
The bandwidth of the video detector chains used with these instruments is limited due to the low currents used in the electron beam. The scan amplitude is usually found to be scan frequency dependent. Reducing the scan rate to match the video bandwidth available results in calibration error at the higher scan rates.